In recent years, consideration has been given to linkage between an automobile navigation system and vehicle control. For example, propositions have been made to control the direction of projection of light from the headlamps of a vehicle based on curve information acquired from map data in an automobile navigation system. When vehicle control is carried out according to road conditions based on map data in an automobile navigation system, as mentioned above, it is required to enhance the accuracy of the map data. Meanwhile, a system has been proposed for the maintenance of road data retained at a central facility so constructed as to retain road data. (Refer to Patent Document 1, for example.)                Patent Document 1: JP-B1-3496479 (U.S. Pat. No. 6,154,152)        
However, the system disclosed in Patent Document 1 involves a problem. This system is intended to add new non-retained road data different from retained road data accumulated at a central facility, and it is not intended to update retained road data itself already retained at a central facility. For this reason, the accuracy of existing road data cannot be enhanced.